Sifting screen

ABSTRACT

A screen for use in a vibratory machine for separating particulate material from liquid material comprising a rigid rectangular support frame having flanges along all four edges to which screen cloths of the type described are bonded with the longer dimension of the rectangular openings defined by the warp and weft wires of the cloth parallel to the longer dimension of the frame. The frame includes an array orthogonal rigid interstices defining a plurality of similar sized rectangular openings or window and the cloths are also bonded to the interstices for the purpose of maintaining the length to width ratio of the openings in the cloths defined by the warp and weft wires thereof. The warp and weft wires of the cloths are tensioned before being bonded to the flanges and interstices.

FIELD OF INVENTION

This invention concerns screens such as are fitted to vibratingscreening machines, sometimes known as shale shakers used to separatesolids from fluids. Such machines are of particular application in theoil well drilling industry to separate drilling mud from base fluidafter recovery from down-hole during drilling.

BACKGROUND TO THE INVENTION

U.S. Pat. No. 5,944,197 describes woven wire screencloth having aplurality of parallel warp filaments intersected by a plurality ofparallel weft filaments configured with a greater number of warpfilaments than weft filaments per given area so as to form rectangularopenings in the weave. Each opening therefore has a length and widthdimension and cloth having such rectangular openings has been found topossess certain desirable properties in relation to the filtering ofcertain muds.

According to U.S. Pat. No. 5,944,197 the length to the width ratio ofeach rectangular opening is optimally in the range 2.7 to 2.8 and anoptimal ratio of the length of each opening to the diameter of the weftfilaments has been found to lie in the range 5.5 to 5.7. This U.S.Patent also teaches that in order to take advantage of the rectangularopenings in the mesh, the interstices must be locked to prevent the warpand weft wires from slipping during operation. Therefore by calenderingthe wire cloth between a set of rollers, the filaments are compressed atthe intersections of the warp and weft and this assists to resistmovement between the warp and weft filaments. Combining optimal wirediameter ratio and calendering, permitted an increased length to widthratio to be employed in the cloth.

Ideally screen conductance is optimised and in general this occurs whenwire diameters for the warp and weft filaments are as small as ispractical. However the desire for increased conductance can have anegative impact on the quality of the screen and its effectiveness andusing smaller diameter warp and weft filaments can result in the warpand weft not remaining perpendicular. Furthermore reducing wire diametercan reduce useful screen life due to wear and tear and breakage of thesmall diameter wires. The mesh provided by the teaching of U.S. Pat. No.5,944,197 has been shown to have a greater conductance and improved lifeover prior art meshes when used to make similar screens, since byemploying slightly larger diameter wires but omitting some of the weftwires, so as to produce rectangular openings in the mesh, there is atrade-off and the reduced conductance is offset by the increased screenlife relative to prior art mesh constructions.

Correct calendering of the cloth assists in preserving the orthogonalityof the warp and weft filaments and maintaining the high ratio of lengthto width in each of the rectangular openings in the weave. Howeverinsufficient calendering pressure fails to lock the warp and weft wiresso that the orthogonality is compromised and the desirable openingratios altered. Too much calendering pressure excessively deforms thewires resulting in shortened operational life. Consequently manufactureof the cloth described in U.S. Pat. No. 5,944,197 has to be the subjectof tight tolerances and close control. Even correctly calendering thewire cloth deforms the warp and weft filaments which when other screenlife-shortening conditions are present can result in earlier failurethan otherwise might be the case.

The present invention seeks to provide a woven wire cloth screen inwhich the openings between warp and weft filaments are rectangular andthe length to width ratios are maintained, thereby retaining the goodconductance characteristics of such a cloth, but which does not requirethe cloth to be calendered.

Since the invention involves the use of wire cloth which is woven inaccordance with the techniques described in U.S. Pat. No. 5,944,197(albeit without being calendered), references to such cloth will bereferred to as cloth of the type described. It is to be understoodhowever that the reference to such cloth is to a wirecloth which hasbeen produced without the step of calendering the cloth after weaving,and is a reference to the choice of warp and weft material, sizes,length to width ratios of the openings in the cloth, and opening lengthto weft filament diameter ratios.

SUMMARY OF THE INVENTION

According to the present invention there is provided a screen for use ina vibrating machine for separating particulate material from liquidmaterial, comprising a rigid rectangular support frame having flangesalong all four edges to which screen cloth of the type described isbonded, and within the frame, an array of orthogonal rigid intersticesdefining a plurality of similar sized rectangular openings below thecloth stretched across so as to be tensioned and bonded to the frame,the cloth also being bonded to the edges of the interstices over whichthe cloth is stretched to maintain both the tension in the wire and thelength to width ratio of the openings in the mesh defined by the warpand weft wires of the wirecloth.

Two (or more) such cloths of the type described may be stretched overthe frame and bonded to the boundary flanges and interstices within theframe, so as to form a two (or more) cloth layer screen.

The mesh size of one of the cloths may be different from that of theother and typically a coarser mesh cloth is laid over the frame first,and a cloth having a smaller mesh size is laid over the coarser meshcloth, and the two cloths are bonded to the flanges and interstices ofthe frame.

In manufacture, one cloth may be laid over and secured first and anyadditional cloth or cloths subsequently laid over and bonded in turn.

Alternatively and more preferably however, the two or more cloths whichare to be bonded to the frame are laid over the frame simultaneouslywith the coarser mesh cloth below the finer mesh cloth or cloths, andthe cloths are gripped and pulled in tension so that differentialtension exists in the cloths at least due to different diameters of thewires forming the cloths, and while the tension is maintained in thecloths, heat and pressure is applied so as to bond the cloths to theedge flanges of the frame and to the interstices within the frame, andthe tension generating pulling forces are only removed after the bondinghas been completed, so that the tension remains in the warps and weftsof the cloths, and the length to width ratios of the openings in themesh of the cloths is preserved by the bonding of the cloths to theinterstices and flanges of the frame.

The invention therefore also provides a method of making a screen foruse in a vibratory machine for separating particulate material fromliquid material comprising the steps of laying screen cloths of the typedescribed over a rigid rectangular support frame having flanges alongall four edges and an array of orthogonal rigid interstices defining aplurality of similar sized rectangular openings within the area definedby the four flanges, arranging the cloths so that the longer dimensionsof the rectangular openings in the cloths are parallel to the longerlength of the rectangular frame, tensioning the cloths in both warp andweft directions, bonding the tensioned cloths to the flanges to securethe cloths to the frame and maintain the tension in the areas of thecloths bounded by the four flanges after forces creating the tensions inthe cloths are removed, and bonding the cloths to the interstices tomaintain the relative positions of the warp and weft wires in the clothsbetween the interstices, and thereby maintain the length to width ratioof the openings in the cloths defined by the interwoven warp and weftwires thereof.

The tension imparted to the cloth in contact with the frame ispreferably different from that in the cloth which overlays it. Likewisethe mesh size of the lower cloth may be coarser than that of theoverlying cloth(s).

Where a thermoplastics material is employed to make the bond, thisentails heating the plastics material at least in the region of the edgeor surface of the frame part to which the cloth or cloths are to bebonded and thereafter allowing the plastics material to cool to hold thecloth or cloths in position.

Whilst the cloths of the type described may be secured to a metalframework defining the edge flanges and orthogonal interstices, in whichthe metal is coated liberally with a suitable plastics material whichupon being heated will accept the cloth and on cooling will bond thecloths to the framework, the invention is of particular application to aso-called composite frame in which at least the four outside members andthe interstices are formed from an array of metal wires and/or rodsencapsulated within a plastics material, preferably a glass reinforcedresin material, such as is described in UK Patents 2322590 and 2337019.

The invention thus provides a screen for use in a vibratory machine forseparating particulate material from liquid material comprising a rigidrectangular support frame having flanges along all four edges to whichscreen cloths of the type described are bonded with the longer dimensionof the rectangular openings defined by the warp and weft wires of thecloth parallel to the longer dimension of the frame, and wherein theframe includes within the four flanges an array of orthogonal rigidinterstices defining a plurality of similar sized rectangular openingsor windows and the cloths are also bonded to the interstices for thepurpose of maintaining the length to width ratio of the openings in thecloths defined by the warp and weft wires thereof, and wherein the warpand weft wires of the cloths are tensioned before being bonded to theflanges and interstices.

Preferably before a first coarse mesh cloth is bonded to the frame twofurther woven wirecloths are laid over on top of the other cloth and allthe wirecloths are individually tensioned and thereafter bonded to theflanges and the interstices under pressure in a single step, the meshsize of the two further cloths being smaller than that of the firstcloth, and differing slightly as between one and the other of the twofurther cloths

It has been discovered that by tensioning cloths of the type describedacross a frame to which they are to be bonded peripherally and along thelines of the interstices of the matrix of struts defining the largenumber of rectangular openings within the frame, the calendering stepcan be omitted without compromising the orthogonal integrity of the warpand weft wires. A consequent advantage is that in the absence ofcalendering there is less chance of deformation and weakening of thecloth interstices, the screen cloth life is considerably increased, andthe manufacture of the cloth is simplified.

By eliminating the calendering step, not only is a processing stageomitted in the production of a screen which requires significantcontrols to be in place, but so is the attendant cost and delay. Theresulting screen therefore possess an economic production advantage aswell as longer life of the cloth meshes stretched thereacross, since theknuckles of the cloth have not been subjected to a calendering stepwhich even when correctly performed can work hard and weaken the warpand weft filaments.

The present invention represents a surprising departure from theinvention claimed in U.S. Pat. No. 5,944,197, since the applicants havediscovered that the calendering step can be omitted without loss oforthogonal integrity of the warp and weft wires in the cloth, if thecloth is bonded to a frame of the type described, in the mannerdisclosed herein.

In particular the applicants have discovered that by using anuncalendered cloth otherwise woven in accordance with U.S. Pat. No.5,944,197, and bonding the cloth to a GRP frame containing internalsteel reinforcements as described and claimed in the aforementioned UKPatent specifications, the cloth interstices become locked as if thecloth were calendered. The resulting screen has the lightweight but highstrength and rigidity properties of screens associated with the frameconstruction described in the aforementioned UK Patent specifications,but with the enhanced conductance associated with cloths woven inaccordance with U.S. Pat. No. 5,944,197, and an increased screen life ascompared to screens employing calendered cloth.

The invention will now be described by way of example with reference tothe accompanying drawings in which:

FIG. 1 illustrates to an enlarged scale part of a cloth woven inaccordance with U.S. Pat. No. 5,944,197 before the calendering step;

FIG. 2 illustrates the cloth after the calendering step proposed by U.S.Pat. No. 5,944,197 and which is omitted when making the cloth for use inthe screen proposed by the present invention;

FIG. 3 is a perspective view of a wire reinforced moulded GRP frame overwhich woven wirecloths are to be stretched and bonded thereto to makethe screen of the present invention;

FIG. 4 is a perspective view of one corner of a jig for tensioning up to3 layers of wirecloth over frames such as shown in FIG. 3;

FIG. 5 is a similar view to that of FIG. 4 in which a coarse mesh clothhas been laid over the frame and tensioned and thereafter a fine meshcloth has been similarly laid over the first and tensioned, each clothbeing of the type shown in FIG. 1;

FIG. 6 is an enlarged end view of the clamping and tensioning mechanismof the jig of FIG. 4;

FIG. 7 is a view of the front of the jig of FIG. 4;

FIGS. 8-12 are side views of a complete apparatus containing a cradle onwhich the jig is mounted, for moving the latter between a tensioningstation and a bonding station and back again to allow the finishedscreens to be removed;

FIGS. 13A and 13B show the clamping and tensioning devices advanced (in13A) ready to receive the edges of the cloths and fully retracted (in13B), retraction serving to tension the cloths gripped between the jawsof the devices;

FIGS. 14-18 are schematic diagrams of the controls and devices forgripping and tensioning the cloths; and

FIGS. 19 and 19A show the control panels to an enlarged scale.

FIGS. 1 and 2 correspond to FIGS. 2 and 6 of U.S. Pat. No. 5,944,197,and FIG. 1 illustrates an enlarged view of part of a piece of wireclothwhich is to form a screen which includes a plurality of parallel warpwires, such as at 12′, 14′, 16′ and 18′ which are crossed by andinterwoven with a plurality of parallel weft wires such as at 22′, 24′,26′ and 28′ at their intersections. FIG. 1 shows the rectangularopenings between the warp and weft wires which are maintained in theuntensioned cloth making up the screen described in U.S. Pat. No.5,944,197 by calendering the wirecloth after weaving and before it issecured to a support as described elsewhere in U.S. Pat. No. 5,944,197.

It can be seen that there are a greater number of warp wires 12′, 14′,16′ and 18′ than weft wires 22′, 24′, 26′ and 28′ per unit or givenarea. The woven warp and weft wires form a plurality of intersectionswhich, in turn, form rectangular openings, such as openings 32′ 34′ and36′. The rectangular openings have a length dimension L and a widthdimension W.

By employing rectangular openings 32′, 34′ and 36′ a greater open areais obtained than with square openings. At the same time, the screen warpwires 12′, 14′ 16′ and 18′ effectively block or screen solid particleshaving a diameter larger than the space between the warp filaments 12′,14′, 16′ and 18′.

The length of the rectangular openings L to the width of the rectangularopenings W may be expressed as a ratio. The length to width ratio whichis optimal for each opening has been found to be between approximately2.7 to 2.8.

In accordance with U.S. Pat. No. 5,944,197 the method of making thescreen involves calendering the top or uppermost screen cloth between aset of rollers. Calendering the screen cloth compresses the cloth at thewarp and weft intersections. This serves to discourage movement betweenthe warp and weft wires and assist in locking the intersections of thewarp and weft wires in place.

FIG. 2 shows a top view of a screen cloth 74′ after completion of thecalendering process. The intersections, or knuckles, such as 76′ and 78′are flattened by the rollers. Additionally, although not visible, wherethe warp and weft wires intersect and touch each other, the wires indentor conform slightly to each other.

It has also been determined that the length of each opening to thediameter of the weft wires may be expressed as a ratio. The optimalration of the length of each opening to the diameter of the wires is aratio of between approximately 5.5 to 5.7.

In particular FIG. 2 shows how the calendering step flattens and, withthe indentation on the crossing surfaces, weakens the wires where onecrosses another at points of intersection, which is avoided by thepresent invention,

FIG. 3 illustrates the relationship of the length of the openings L tothe diameter of the weft wires d. The relationship may be expressed asfollows:L/d=5.5-5.7

Where “L” is the length of the opening and “d” is the diameter of theweft wires. The length of the opening “L” is the actual opening and notthe distance between centre of one weft wire to the centre of anadjoining weft wire.

The following Figures in the drawings illustrate a method ofmanufacturing a filter screen according to the invention and apparatusfor manufacturing such a screen. The filter screen can be used as afilter in vibrating filtration equipment such as shakers which are usedin the oil drilling industry for separating solids from the liquid phaseof oil and water based muds retrieved from drilling operations. Such ascreen is described in Patent Specification No WO95/23655. This screenhas upper and lower wire mesh cloths of differing qualities stretchedover a metal rod reinforced frame of plastics material to which thecloths, after tensioning, are bonded by adhesive. Later PatentSpecification No. WO98/37988 describes a frame to which the tensionedcloths can be bonded by heat softening the surface of the frame andpressing relevant parts of the cloths into the softened plasticsmaterial, the bond being completed by then allowing the plasticsmaterial to cool and cure.

The method and apparatus described herein provides a method ofmanufacturing a filtering screen according to which a frame of plasticscoated metal or reinforced plastics material is located in a jig, atleast one wire cloth is placed over the frame and its edges are grippedby pneumatically operated clamps carried by the jig, the clamps arepneumatically driven outwardly of the frame to tension the cloth inorthogonal directions, the tensioned cloth is pressed against the frameby a heated platen in order to bond the cloth to the frame and aftercompletion of the bonding step the clamps are released, the frame isremoved from the jig and where necessary the cloth trimmed back to theedges of the frame.

The apparatus comprises a jig for locating at least one such frame ofplastics coated metal or reinforced plastics material, pneumaticallyoperated clamps carried by the jig for gripping the edges of a metalwire cloth placed over the at least one frame, pneumatically driventensioning means on the jig for driving the clamps outwardly from theframe to tension the cloth in orthogonal directions, a platen, means forheating the platen, means for driving the heated platen and/or the jigto press the cloth against the frame in order to locally melt theplastics material and allow the cloth to become embedded therein, andthen to retract the platen and/or jig to allow the at least one frameand cloth to cool in order to bond the cloth to the frame, and to allowthe frame then to be removed from the jig after release of the clamps.

Two or three layers of wire mesh cloth can be bonded to the frame.

Typically a lower cloth of coarser mesh is located below at least onefiner mesh cloth, and preferably two finer mesh cloths having slightlydiffering mesh sizes are laid over the coarser mesh cloth, and all threeare bonded to the frame.

The plastics material of the frame is typically polypropylene,

The frame itself has a rectangular periphery with a lattice oforthogonal intersecting bars. During the heating step, the plasticsmaterial forming the upper surface of the frame is softened and thetensioned cloths are pressed into the softened plastics material andbecome embedded therein. The bond is completed by allowing the frame tocool and the plastics material to cure. It is possible for the uppersurface of the frame to be ridged to aid the bonding step, in that it isthen only necessary for the heated platen used to press the clothsagainst the frame, to soften the ridges which then accept the wire meshto form the bond.

It is most desirable to protect the cloth(s) from direct contact withthe heated platen, and for this purpose a heat resistant non-stickfabric, such as PTFE glass fabric, is preferably laid over the clothsprior to the bonding step. The fabric is removed on completion of thebonding step.

When two or more wire cloths are to be incorporated into the screen, aseparate set of pneumatically operated clamps is provided for eachcloth. Similarly, a cloth tensioning mechanism is provided for theclamps along at least two adjacent sides, and preferably all four sides,of each cloth. The cloths can thus be individually and differentiallytensioned as appropriate for the use to which the screen is to be put,prior to being collectively bonded to the frame.

Each clamp comprises a fixed pair of jaws and an inflatable envelopebetween them for use in gripping the edge of the wire cloth. A thinaluminium plate separates the envelope from the cloth. When inflated,the envelope presses the aluminium plate towards one of the jaws, whichis preferably lined with a rubber (or like material) strip, typically 3cm wide, to improve the grip on the edge of the cloth sandwiched betweenthe plate and the jaw.

A control panel is provided to enable a jig operator to activate thepneumatic clamps and the pneumatic tensioning means. Preferably fourcontrol panels are provided, so that the operator can feed the clothinto the clamps along one side of the frame and activate these clamps toclamp that edge of the cloth using one control panel, before moving toanother side of the fame and repeating the procedure for that side ofthe frame, and so on. Where there are two or more levels of clamps andtensioning mechanisms, one level for each layer of cloth, the firstcloth is individually clamped and tensioned on all four sides, and thenthe next, in a similar manner until all the cloths have been clamped andtensioned as appropriate.

The clamping and tensioning of the cloths is performed at a firststation remote from a second station at which the bonding occurs.Pneumatic power for the clamps and tensioning mechanisms is thereforesupplied to the jig through a flexible supply cable, in order to allowfor the movement of the jig between the stations.

Initially, therefore, the at least one frame is loaded into a jigcarried by a cradle which, after the cloths have been clamped andtensioned, is conveyed to the second station containing the heatedplaten.

The cradle preferably has wheels running on rails along which the cradleis driven between the first and second stations by a linear drive,conveniently a Festo® linear pneumatic drive.

The platen is preferably pre-heated and when in position, the jig cradleis raised up to the platen by a hydraulic ram, to commence the bondingstep. Thus, the ram may act against the underside of the cradle to liftit off the rails on which the cradle is driven between the first andsecond stations.

Where a non-stick fabric is to be placed on top of the cloth before itis engaged with the heated platen, the fabric may be placed on the clothat the first or second station, or in transit.

The platen is typically pre-heated to a temperature in the range 200 to300 degrees C., preferably about 250 degrees C.

The hydraulic pressure to the ram is adjustable in the range 500 to 2000psi. to suit the screen materials, and in particular the cloth or clothcombinations used.

The hydraulic pressure to effect the squeeze and heating and bondingstep is typically applied for a period of time in the range 30 secondsto 2 minutes.

At the end of the heating and bonding step, the jig cradle and platenare separated to allow the material to cool and cure, and finally thejig cradle is conveyed back from the second station to the clamping andtensioning station, to allow the operator to release the clamps, removethe protective fabric, remove the frame or frames from the jig cradle,and trim the cloths extending beyond the edges of the frame(s).

In order to speed up manufacture two frames are covered at a time,arranged side-by-side, and large area pieces of wire cloth, one for eachlayer, are stretched over both frames. After the bonding step the largearea(s) of cloth are cut along a line between the two side-by-sideframes. It will be appreciated that the jig cradle is thus relativelylarge, but the time taken to fit, clamp and tension each large sheet islittle different from what would be required to similarly handle smallerareas of cloth over a single frame, and each heating and bonding stepresults in two screens instead of just one.

The clamps of any one layer are preferably movable outwardly of theframe individually and independently of each other, i.e. each clamp hasa spearate associated tensioning mechanism, so that any localised slackin the cloth can be taken up.

A breaker bar may be located immediately in front of each line of clampseach of the sides of the frame, and the cloth(s) move over it as thecloth(s) are tensioned, thereby to remove any ripples in the cloth.

Referring to FIGS. 3 to 13, the illustrated apparatus comprises asupporting framework 10 which at one end supports a cradle 12 forming ajig for enabling assembly of screen frames at an assembly station 14,and which at the other end is integrated with a heating platen supportunit 16 constituting a bonding station 18. The cradle 12 has wheelswhich run on parallel spaced apart rails 22, 22A to enable the cradle tobe moved between the two stations by a drive 23. (see FIG. 10). At leastone rail and the wheels corresponding therewith are designed to ensurethat the cradle moves in a straight path.

The drive includes a pin 25 which engages in a groove 27 in plate 29,which latter is secured to and extends from the cradle (see FIG. 10).The drive 23 is a Festo® linear pneumatic drive.

A flexible services umbilical cord 24 conveys electrics and pneumaticsto the cradle.

At the screen assembly station, two GRP screen support frames 26, one ofwhich is shown in FIG. 3, are laid side by side in the jig cradle 12.FIG. 4 shows one of the frames 26 located in the cradle 12, with thespace for a second frame 26 beside it. The plan view of the apparatus(FIG. 12) also shows one frame located in the jig. Each frame 26comprises a rectilinear grid of orthogonally intersecting, wirereinforced glass fibre reinforced polypropylene bars as described inPatent Specifications WO95/23655 and WO98/37988. In known manner, theupper surfaces of the peripheral edges and intersecting bars of theframe are moulded with upstanding ridges.

Having located the two frames in the jig woven wire filter cloths arelaid in turn over the frames, and the edges of each cloth inserted intoand gripped by pneumatic clamp units 28 carried by the jig before thenext cloth is applied. The pneumatic clamp units are shown in moredetail in FIG. 7. The warps and wefts of the cloths define rectangularopenings and the cloths are laid over the frames with the longerdimensions of the rectangular openings in the cloth parallel to thelonger edges of each frame.

Each clamp comprises fixed upper and lower jaws 28A, 28B the lower ofwhich carries on its upwardly facing surface an inflatable envelope 28Cover which, lies a protective aluminium plate 28D. A cloth edge isinserted between 28D and a rubber strip 28E on the underside surface ofthe upper jaw, so that when 28C is inflated the cloth is clamped betweenplate 28D and the upper jaw, the rubber strip 28E reducing the risk ofslip occurring when the cloth is tensioned. In use, the edge of a wirecloth is pulled over the breaker bar 31 and inserted between the jaws28A, 28B between the rubber strip and the aluminium plate. The envelopeis then inflated to grip the cloth between the rubber strip and theplate with a pressure sufficient to withstand the tensioning forcessubsequently to be applied to the cloth.

As best seen in FIGS. 13A and 13B the rear edge of each aluminium plate28D is upturned to form a lip which engages in a groove in the undersideof the upper jaw 28A near the rear edge thereof.

In order to enable cloth tensioning, jaws 28A, 28B are joined at theirrear to form a single assembly carried by the piston 28F of a doubleacting pneumatic cylinder 28G. The double acting pneumatic cylinder andpiston 28G, 28F constitute a tensioning device and there is one suchdevice for each clamp unit. Supplying air to the forward end of thecylinder 28G forces the piston rearwardly, and thereby the jaw assembly,back away from the jig to stretch the gripped cloth over the breaker bar32 at the periphery of the jig. Supplying air to the rear end of thecylinder moves the jaw assembly forward to its rest position nearer thejig as shown in FIG. 13A. This is effected after the cloth has beenbonded to the frame and the jaws released, as later described. Thebreaker bar 32 prevents any local rippling of the wire cloth duringtensioning.

In practice, it will be understood that all four edges of a wire clothare gripped prior to tensioning, as will be clear from laterdescription.

The pneumatic supply for the clamp units is later described withreference to FIGS. 14A and 14B.

Three layers of clamp units 28 are provided, to allow up to three clothsto be laid over and secured to the frames.

Three lines of clamp units 28 (such as can be seen in FIGS. 7, 13A and13B) are provided on all four sides of the cradle, and a master controlpanel 30 (see FIGS. 5 & 6) is provided at one corner, and three slavecontrol panels 30A, 30B and 30C are located at the other corners, sothat an operator can control the clamps and tensioning devices alongsuccessive sides of the cradle individually, by simply moving from oneside to the next therearound.

Having gripped all four edges of a wire mesh cloth between the clampunit jaws, the latter are pneumatically driven outwardly of the jig bymeans of the aforesaid cylinder and piston devices, in order to tensionthe cloth over the two frames. Each tensioning device, constituted bythe aforesaid double acting pneumatic cylinder and piston, is operableindependently, to stretch the cloth over the breaker bars 32 extendingalong the four edges of the jig cradle, whereby to ensure that the clothis uniformly tensioned without ripples or creases.

The process is repeated for each cloth, a first coarse mesh cloth beinglocated between the jaws of the lowest line of clamps 28, a finer meshcloth in the next line of clamps, and another fine mesh cloth in thethird line of clamps, the mesh size of the two fine mesh cloths beingsimilar but not identical.

The complete cradle can then be moved to the bonding station, under theheating platen on the unit 16, best seen in FIGS. 9 and 12. First,however, before bonding, in order to prevent direct contact between theupper screen cloth and the heating platen, a heat resistant non-stickfabric, e.g. PTFE glass fabric, is laid over the tensioned cloth in thejig cradle. If desired the edges can be gripped by a line of clampunits. FIG. 5 shows the jig 12 with the two frames 26 located therein,upper and lower wire mesh cloths 26A and 26B cut away in part, stretchedover the frame, and the release fabric 26C, also cut away in part, laidover the cloths.

At the bonding station, the jig is raised and lowered by a platform 31acted on by a hydraulic ram 34, the platform 31 engaging the undersideof the jig 12 to lift it, and in so doing lifting the wheels 33, 35 offthe rails 22, 22A. On being raised the cloth on the screens is broughtinto contact with the platen 36, which is pre-heated by an electricheating element (not shown).

The heated platen softens the ridges on the upper edges of theinterstices and the side flanges of the plastics frames, and the warpand weft wires of the tensioned cloths are pressed into the softenedmaterial. The ridging of the side flanges, and interstices aids thebonding process, as described in the aforementioned publishedInternational Patent Specifications.

After bonding the screen assembly is then lowered and moved back to theassembly station.

Back at the assembly station, the PTFE release fabric is stripped off,the tension on the cloths is released, the screen cloths are cut betweenthe two frames, each frame is removed from the jig, and the protrudingedges of the wireloth are trimmed back to the edges of the frames by forexample using an angle grinder. Manufacture of the screens is thencomplete, and the jig is now ready to receive the next two frames andlayers of cloth to make the next two screens. The bonding of the clothsto the interstices as well as to the edges of each frame serves tomaintain the shape and length to width ratios of the warp and weftdefining openings in the cloths.

For completeness, FIGS. 14A and 14B show the air supply lines to theclamp units. Air for inflating the envelope 28C in the jaws 28A, 28B issupplied through air line 40. Complete evacuation of the envelope isrequired fully to open the space between the jaws prior to insertion ofthe edges of a fresh wire mesh cloth and this is achieved by applyingvacuum to line 40.

Air for operation of the double acting cylinder 28G is supplied throughair lines 42, 44. Supplying air through line 42 drives the piston 28F,and thus the clamp, outwardly of the jig in order to tension a wire meshcloth clamped between the jaws. Supplying air through line 44 drives theclamp back towards the jig at the end of the operational sequence, sothat the clamps are back in the positions shown in FIG. 13A ready toreceive the next screen cloths for bonding to the next pair of frames.

The pneumatic control circuit for the clamps and tensioning devices isshown in FIG. 12.

Air under pressure is supplied via an on/off valve 46 to a pressureadjustable valve 48 and associated pressure meter 48A and thence througha filter and valve unit 49 to a pressure line 50 from which air can besupplied via a pneumatic switching circuit 52 to operate the clamps andtensioning devices. The switching circuit is controlled by buttons onthe control panels at the corners of the jog, those buttons beingrespectively labelled L3, L2, L1 and P1 in the drawing. Each linepressure for the clamp and tensioning devices is adjustable by means ofpressure adjusters and associated meters 54. A typical line pressure P1for operating the clamps may be 5.5 bar, and typical tensioningpressures may be 4.8 bar (L3) for the uppermost layer of tensioningdevices, 3.0 bar (L2) for the intermediate layer and 2.0 bar (L1) forthe bottom layer. There may, of course, be only one or two wire meshcloths applied in the manufacture of any particular screen, in whichcase some of the buttons for the unused layers of tensioning deviceswill not be operated. The exemplary line pressures mentioned for thetensioning devices are given assuming all three layers of tensioningdevices are to be used. The valves employed in the pneumatic controlcircuit are all solenoid valves.

FIGS. 13 and 14 show the set up of the pneumatics for the clamp units.First, it has to be understood that the frames to which the wire meshcloths are to be bonded are rectangular, with two shorter sides A and Band two longer sides C and D. The two frames are located in the jig withtheir shorter sides A and B in line, thus requiring six clamp unitsalong each of the aligned edges, (twelve in all) for a cloth laid overboth frames. Four clamp units are provided along each longer side of theframes at opposite ends of the jig (eight in all).

FIG. 13 shows the set up for six clamp units along one side of the jig;this set up is repeated for the other side.

FIG. 14 shows the set up for the four clamp units along the longer endsof the jig. The set up of FIG. 14 is also repeated at the opposite end.In both figures, the set up is shown for the clamp units in all threelayers.

In both of FIGS. 13 and 14 switching valves 56 control the inflation andcollapse of the envelopes in the clamps through lines 58 and 60. Ventingthe envelopes to atmosphere via 60 releases the cloths to allow theiredges to be removed. Applying vacuum to 60 by using 62, collapses theenvelopes to facilitate the insertion of new cloth between the clampingjaws. Valve units 64 control the supply of air to the pneumatic cylindertensioning devices through lines 64, 65, line 64 for tensioning and line65 for untensioning.

In FIG. 15, reference 68 indicates the fused electrical power inputterminals; reference 69 indicates the power supply to the motor drivingan oil pump for supplying hydraulic oil under pressure to the ram whichlifts and lowers the cradle at the bonding station; reference 70indicates the power supply to a photoelectric safety circuit which isprovided to stop and reverse the hydraulic drive if a light beam in thepath of the cradle across the entrance to the bonding station isinterrupted for any reason while the cradle is being lifted towards theheating platen; reference 71 indicates the power supply enablingcomputer control of the sequence of the operating procedure of theapparatus; reference 72 indicates the power supply for all AC solenoidvalves employed in the pneumatic circuits and reference 74 is the powersupply for all DC solenoid valves, and reference 76 indicates the powersupply to the heater used in heating the platen at the bonding station.

Finally FIGS. 16 and 16A respectively show the master control panel 30at one corner of the jig, and one of the secondary (or slave) controlpanels 30A, 30B, 30C at one of the other corners of the jig. The buttonsare marked with their functions.

The main panel 30 and each secondary panel 30A, 30B, 30C provides forclamping, unclamping and evacuation of the clamp envelops, along thefirst, second, third and fourth sides of the jig respectively, ifappropriate for all three layers of cloth.

The main control panel 30 additionally provides for tensioning anduntensioiling along all four sides of the jig, including the first side,which is where the operator starts the loading of the jig. Thus, theoperator, having first located the edge of a cloth in the clamps alongthe first side of the jig, uses the main control panel 30 to operatethose clamps to clamp that edge, and then moves round the jig to clampthe edges of the cloth in sequence on the second, third and fourthsides. This brings the operator back to the main control panel, at whichthe appropriate tensioning button can be pressed to operate all thepneumatic cylinder tensioning devices simultaneously so all the clampsfor one layer are moved outwardly at the same time, thus tensioning thecloth appertaining to that layer.

When loading has been completed and the cradle moved back to theassembly station, the operator can use the main control panel 30 firstto apply pressure to reverse all the tensioning pneumatic cylinders ofall the cloth layers.

All the clamping can be released by evacuating all the inflatedenvelopes to atmosphere, and the latter can be flattened to assist ininserting more wire cloth edges, by applying vacuum to all theenvelopes.

It is important to note that, during tensioning, the same air pressureis applied to the pistons of the pneumatic tensioning devices and thesemay not all move exactly equally, but each will move to the extentnecessary to tension all parts of the relevant cloth to thepredetermined and preset extent.

1. A method of making a screen for use in a vibratory machine for separating particulate material from liquid material, comprising laying over a rigid rectangular support frame, which has flanges along all four edges, a wire cloth having a greater number of warp wires than weft wires per given area, so as to define a plurality of rectangular apertures each having a length to width ratio of between approximately 2.7 to 2.8, the ratio of said length to the diameter of the wire being approximately 5.5 to 5.7, characterised by the steps of providing the frame with an array of orthogonal rigid interstices defining a plurality of similar sized rectangular apertures within the area defined by the four flanges; arranging the wire cloth so that the longer dimensions of the rectangular apertures in the cloth are parallel to the longer length of the rectangular frame; tensioning the cloth in both warp and weft directions; bonding the tensioned cloth to the flanges to secure the cloth to the frame so as to maintain the tension in the areas of the cloth bounded by the four flanges; removing the forces creating the tension in the cloth; and bonding the cloth to the interstices to maintain the relative positions of the warp and weft wires in the cloth between the interstices, thereby to maintain said length to width ratio of the apertures in the cloth defined by the interwoven warp and weft wires thereof, without the step of calendering the cloth:
 2. A method as claimed in claim 1 wherein a second cloth is laid over the first mentioned cloth, and the tension imparted to the cloth in contact with the frame is different from that in the second cloth.
 3. A method as claimed in claim 2 wherein the cloth in contact with the frame has a coarser mesh than that of the second cloth.
 4. A method of making a screen as claimed in claim 2 wherein one cloth is laid over and bonded first, and any additional cloth is subsequently laid over and bonded to the frame in turn.
 5. A method of making a screen as claimed in claim 1 wherein two or more cloths which are to be bonded to the frame are laid over the frame and gripped and tensioned one after the other, and while the tension is maintained in the stretched cloths, heat and pressure is applied so as to simultaneously bond the cloths to the edge flanges of, and to the interstices within, the frame, and forces generating the tensions are only removed after the bonding has been completed, so that when the forces are removed, the tensions remain in the warps and wefts of the cloths.
 6. A method of making a screen as claimed in claim 1 wherein a thermoplastics material is employed to make the bond, and this is achieved by heating the plastics material at least in the region of the edge or surface of the frame part to which the cloth or cloths are to be bonded, thereafter allowing the plastics material to cool and thereafter anchor the cloth or cloths in position on the frame.
 7. A method of making a screen as claimed in claim 1 wherein a metal frame is employed and prior to overlaying the frame with the wire cloth the metal framework is coated liberally with a thermoplastics material which upon being heated softens and allows the cloth to become embedded therein and on cooling bonds the cloth to the framework.
 8. A method of making a screen as claimed in claim 1 wherein the frame is moulded from a plastics material and is reinforced throughout by a pre-assembled array of wires and/or rods, and the screen is formed by heating the surface of the moulding in contact with the cloth(s) stretched thereover to soften the surface regions of the frame into which the cloth(s) become embedded and bonded as the frame is allowed to cool.
 9. A screen when made in accordance with the method of claim 4 wherein the cloths are woven from wire of similar metallic composition and the same force is applied to the different cloths, the tensions in the warps and wefts of the cloths being related to the diameter of the wires making up the cloths.
 10. A screen when made in accordance with the method of claim 4 wherein the cloths are subjected to different tensioning forces so as to establish different tensions in the different cloth layers.
 11. A screen for use in a vibratory machine when made in accordance with the method of claim 1, comprising a rigid rectangular support frame having flanges along all four edges to which the wire cloth is bonded with the longer dimension of the rectangular openings defined by warp and weft wires of the cloth parallel to the longer dimension of the frame, and wherein the frame includes within the four flanges an array of orthogonal rigid interstices defining a plurality of similar sized rectangular openings, and the cloth being bonded to the interstices for the purpose of maintaining the length to width ratio of the openings in the cloths defined by the warp and weft wires thereof, and wherein the warp and weft wires of the cloths are tensioned before being bonded to the flanges and interstices.
 12. A screen as claimed in claim 11 wherein before a first cloth is bonded to the frame at least one further woven wire cloth is laid over the first cloth and both wire cloths arc tensioned and thereafter bonded to the flanges and the interstices under pressure in a single step.
 13. A screen as claimed in claim 12 wherein a different tension exists in the first cloth from that in the cloth which overlies it.
 14. A screen as claimed in claim 12 wherein there are two cloths and the mesh of the first cloth in contact with the frame is coarser than that of the cloth which overlies it.
 15. A screen as claimed in claim 12 wherein there are three cloths, the mesh of the first cloth in contact with the frame is coarser than that of the other two which overlie it, and the mesh of the two overlying cloths is substantially finer than that of the first coarse mesh cloth.
 16. A screen as claimed in claim 15 wherein the mesh size of one of the two overlying cloths is substantially the same as, but not identical to, that of the other.
 17. A screen for use in a vibrating machine for separating particulate material from liquid material, comprising: a) a rigid rectangular support frame having flanges along all four edges; b) an array or orthogonal rigid interstices mounted in the frame to define a plurality of similar sized rectangular openings; c) a wire cloth having a greater number of warp wires than weft wires per given area, so as to define a plurality of rectangular apertures each having a length to width ratio of between approximately 2.7 to 2.8, the ratio of the length to the diameter of the wire being approximately 5.5 to 5.7; d) said cloth being stretched across the frame so as to be tensioned in both warp and weft directions, and being bonded to the frame and to the edges of the interstices over which the cloth is stretched; e) whereby both the tension in the wire and the length to width ratio of said apertures defined by the warp and weft wires of the cloth is maintained without the cloth being calendered.
 18. A screen as claimed in claim 17, wherein at least two wire cloths each as aforesaid are stretched over the frame and bonded to the boundary flanges and interstices within the frame, so as to form a multilayer cloth layer screen.
 19. A screen as claimed in claim 18, wherein the mesh size of one of the cloths is different from that of the other.
 20. A screen as claimed in claim 19, wherein a cloth having a smaller mesh size overlays a coarser mesh cloth, both cloths being tensioned and bonded to the frame and interstices. 